On a pressure map, you can just make out a broad area of low pressure. On the rainfall map, a belt of heavy rain stands out along the Pacific coast, well away from the centre of the low. How do those two pictures fit together? One weather pattern that can explain them is a Central American gyre, or CAG.
Meteorix illustration of the weather pattern.
During the rainy season, winds in the lower atmosphere sometimes organise into a broad counterclockwise circulation over Central America and the surrounding waters. To see its shape, zoom out until southern Mexico, the Central American isthmus and parts of both the Pacific and the Caribbean fit on your screen. Zoom in on a single country and all you might see is a steady wind blowing in from the sea.
A study covering 1980–2010 identified 47 gyres. They occurred most often in two parts of the rainy season, May–June and September–November. Those peaks describe a pattern over three decades. To identify a gyre on a particular day, look at how the winds are circulating.
Picture a mountainside facing the Pacific. As long as the gyre keeps moist ocean air flowing towards it, the terrain forces that air uphill. As the air rises and cools, some of its water vapour condenses. Fresh clouds can form over the same slopes that saw rain just hours earlier. Which slopes face the incoming flow depends on the gyre’s position and the wind direction.
A radar image might show that the heaviest downpour has already moved on. A rainfall map covering several days tells you how much water successive bands have dropped over the same river basin. During a prolonged event, look upstream as well. Rain that fell there counts towards the water flowing through the basin.
Yes. Sometimes a smaller disturbance over water develops its own circulation within part of the gyre. Thunderstorms begin to cluster near its centre. If the system becomes sufficiently organised, it can be classified as a tropical depression. In other cases the gyre continues to bring rain for days without producing a tropical cyclone.
Start with the world pressure map. Zoom out to include the Pacific and the Caribbean and trace how the wind changes direction around the low. Compare the 24-hour and 72-hour rainfall forecasts to see where rain could persist. Then open Advanced Observation and select the 24-hour satellite rainfall estimate to see where rain has already fallen.
Open the Meteorix map ↗ Rainfall forecast ↗ Advanced Observation ↗The 1980–2010 study found two peaks, in May–June and September–November.
For official forecasts and warnings, consult your national weather service.
The axis of a tropical wave can cross an island in the morning and the downpours arrive hours later. To understand the timing, follow both the wind and the clouds.
Yesterday you could see a cluster of thunderstorms on satellite imagery. Today the same cluster has a label, 92L. What changed? The code tells you how forecasters are tracking the disturbance. Its intensity is a separate question.
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